The present invention relates to an intake air flow rate measurement apparatus of the type used to measure the intake air flow rate in an internal combustion engine; and, more particularly, the invention relates to an intake air flow rate measurement apparatus with which that the intake air flow rate can be correctly measured based on the waveform of a signal output from an electrothermal resistor type air flowmeter, such as a hot-wire type flowmeter.
The flow rate of an intake air flow into an internal combustion engine has been generally measured with a hot-wire type air flowmeter. The hot-wire type air flowmeter is arranged in an air intake passage and measures the flow rate of intake air passing the cross section of the air intake passage by measuring the required amount of electric current to be fed to a hot-wire of the air flowmeter in order to maintain the temperature of the wire at a constant value. Since the hot-wire type air flowmeter can measure only the absolute value of an air flow rate, this type of air flowmeter cannot discriminate the direction of the air flow being measured.
In an internal combustion engine, a pulsation is caused in the intake air flow due to the up and down motions of the pistons in the cylinders. Accordingly, the air flow rate at the cross section of the air flow passage at which the hot-wire is arranged indicates periodic changes. If the opening of a throttle valve becomes large, the amplitude of the pulsation also becomes large, and the intake air sometimes temporarily flows in the negative (reverse) direction in the air flow passage. This reverse flow is called a back flow. Therefore, since the hot-wire type air flowmeter can detect only the absolute value of an air flow rate, the air flow rate detected by the hot-wire air flowmeter is not correct when back flow occurs.
Further, the hot-wire air flowmeter possesses a response delay due to its heat capacity. Furthermore, the relation between the value of a signal output from the hot-wire type air flowmeter and the actual air flow rate is nonlinear. If the output signal of the hot-wire type air flowmeter, which is affected by the response delay and this non-linearity, is integrated as it is, the amount of intake air sucked into the internal combustion engine will be overestimated by the amount corresponding to the back flow.
Conventional methods to solve the above problems have been devised.
For example, in a method disclosed in Japanese Patent Application Laid-Open Hei. 10-300544, a threshold value is set based on maximum and minimum values of a signal output from a hot-wire type air flowmeter, the direction of intake air flow in signals output from the air flowmeter is judged using the set threshold value, and an interval of back flow is determined. Further, the correct air flow rate is obtained by correcting the detected signal based on the determined interval of back flow.
Moreover, in a method disclosed in Japanese Patent Application Laid-open Hei. 7-167697, maximum and minimum values of a signal output from a hot-wire type air flowmeter are detected, and the time which has elapsed between detection of the maximum to minimum values is obtained. Then, an interval of back flow is determined based on the elapsed time, and the output signal of the air flowmeter is corrected based on the determined interval of back flow.
Furthermore, in a method disclosed in Japanese Patent Application Laid-Open Hei. 7-239260, a map in which the relationship between values of the flow rate of back flow and the opening of a throttle valve and a map in which the relationship between the flow rate of a back flow and the change amount of the crank angle are prepared in advance, and the output signal of the air flowmeter is corrected using an amount of back flow calculated on the basis of the above maps, the detected opening of the throttle valve and the change amount of the crank angle.
Although the occurrence of back flow is determined by checking whether the output signal of the air flowmeter goes below a threshold value in the method disclosed in Japanese Patent Application Laid-Open Hei. 10-300544, it sometimes becomes impossible due to noise components in the output signal of the air flowmeter to clearly determine whether the output signal of the air flowmeter goes below the threshold value. Therefore, this method is not practically reliable.
Further, although maximum and minimum values of signals output from a hot-wire type air flowmeter are detected in the method disclosed in Japanese Patent Application Laid-Open Hei. 7-167697, pseudo-maximum and pseudo-minimum values are sometimes detected due to noise components in the output signals of the air flowmeter. Therefore, this method is also not practically reliable.
Furthermore, in the method disclosed in Japanese Patent Application Laid-open Hei. 7-239260, it is necessary to prepare a map which defines the relationship between the flow rate of a back flow and the opening of a throttle valve and a map which defines the relationship between the flow rate of back flow and the change amount of the crank angle for each internal combustion engine in advance.
Therefore, if any change is made an air intake system, these maps must be renewed.